NavigationStack has been around since iOS 16, @Observable since iOS 17. and I'm not sure if the author is aware that GeometryReader hasn't been necessary since iOS 18 added onGeometryChange modifiers. Nearly 100% of users are on iOS 16+.
I'm doing some fairly complex work with SwiftUI and I find most components work identically between iPad and macOS. I can work for a week without needing to test on an iPad.
There are problems with SwiftUI for sure: I'm finding macOS performance on an M2 Max is far worse than an iPad M1, especially with animations. Toolbars are very inconsistent between platforms and the compiler timeouts are a real pain (hopefully Swift 6.4 will fix that).
I think the author does not understand how and why to use GeometryReader at all. He literally said "you find yourself wrapping everything in a GeometryReader" which is a different thing from simply measuring. For measuring you could always put GeometryReader into `.background()` and use that to avoid messing with the view layout. Also, "wrapping everything" is a good sign you are doing something wrong.
Edit: by the way compiler timeouts are not a SwiftUI issue: it is a Swift issue. You can create a timeout situation in pure Swift. For SwiftUI specifically this situation is addressed in the upcoming OS27 SDK.